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Len Reid, Vice President, Technology
Aircraft design and assembly challenges include provision of hole protection from lightning strike in fastened joints; especially in the vicinity of fuel tanks or electrical services. Installation of interference fit fasteners helps minimize the effect of lightning strike and also enhances the durability and load transfer effectiveness of the joint. However, interference fit fasteners are known to cause laminate damage during installation which can degrade the fastener system performance over time. Use of a pre-installed expanded grommet into the hole prior to installing interference fit fasteners can greatly enhance the effectiveness of the joint. At the same time they minimize arcing or plasma discharge at the fastener in the event of lightning strike by eliminating the air gap between the grommet and the composite.
The Fatigue Technology (FTI) GromEx® system was designed as basically a thin-walled "bushing" or liner for composites to protect the hole from laminate damage during interference fit fastener installations. The GromEx system is based on cold expanding an initially clearance fit grommet into a hole. A specially sized grommet, with a proprietary lubricant on the inside surface, is radially expanded into the hole using a tapered expansion mandrel. The process of expanding the grommet into the composite hole, at an expansion level below the threshold that could cause damage to the surrounding composite, will yield it into the hole creating an interference fit. An example of a typical GromEx installation is shown. Overall, the expanded grommet installation method provides a rapid, consistent, and non-damaging grommet installation.
FTI completed a test program to evaluate the performance of the GromEx system under lightning strike test conditions. The evaluation was done in accordance with the methods specified in SAE ARP 5416. Results confirmed that eliminating the air gap between the grommet and the composite reduced the probability of arcing, which could lead to ignition of a plasma explosion at the interface during lightning strike. In addition, GromEx enables use of interference fit fasteners to eliminate air gaps thereby further reducing the arcing potential. The combination of GromEx with an interference fit fastener provides a very low resistance path between the fastening system and the structure.
To ensure the expanded grommet system would not cause detrimental damage to the composite structure as a result of the installation, GromEx grommets were installed into composite laminate and then sectioned and microscopically examined for localized damage. The optimum expansion level was determined from a large number of such evaluations in a variety of composite substrates and lay-ups.
In addition to the greatly improved lightning strike performance, other structural benefits are realized by using the GromEx system including:
• increased fatigue life and durability of the joint
• improved static performance of the joint
• allow higher interference fit of the fastener through a stacked joint
• the ability to remove and replace the fastener without causing secondary removal/insertion damage in the composite matrix
• versatility of assembly by allowing a range of different fastener types in the one joint or assembly
• improved sealing of a fuel tank joint assembly
• the ability to "sleeve" a composite metal hybrid joint
FTI has performed extensive mechanical testing of its range of expanded bushing type products in Carbon Fiber Reinforced Plastic (CFRP) to determine the guidelines for their use and to generate a general database for performance of these products in composite materials. From a structural point of view, the expanded grommet enhanced load transfer and improved joint durability compared to the bare fastener installation. In all specimens, fastener head failure terminated the test.
For more information on FTI's GromEx in composite panels and the benefit of lightning strike protection, please visit our online technical library, register, and download our composite technical brief #456289. In addition, a more in-depth description of the GromEx installation process, additional structural benefits afforded by the GromEx system, and the results of lightning strike simulated direct strike tests will be discussed in a FTI paper at the upcoming SAE AeroTech conference to be held in Toulouse, France in October this year. Please check the official AeroTech agenda for the presentation time and location. I will also be available at the FTI booth (#310) and welcome the opportunity to discuss your potential applications.
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Fatigue Technology was one of only 26 suppliers from North America to be honored by Northrop Grumman for its performance in 2010. The Platinum Source Preferred Award recognizes Fatigue Technology as a company that achieved distinction in product quality, on-time delivery, customer satisfaction, and robust lean processes. FTI has now been honored with this award in consecutive years by Northrop Grumman.
FTI Recognized with Northrop Grumman Supplier Award
FTI earned the Boeing Performance Excellence Award (Silver Level) for the fourth year in a row. The Boeing Company issues awards annually to recognize suppliers who have achieved superior performance. FTI maintained a Silver composite performance rating for each month of the 12-month performance period, from October 1, 2009, to September 20, 2010. FTI is among 358 suppliers to receive the Silver level of recognition.
Additionally, FTI was recognized by Bell Helicopter with its prestigious Premier Supplier Award. In order to qualify for the Premier status a supplier must meet numerous standards that include being AS9100 certified, delivering high-quality materials on a 98 percent frequency rate or higher, and score very high on a cross-functional inspection conducted on-site by a specialized Bell team. The award recognizes FTI for their superior business processes and world class quality standards.
FTI Receives Premier Supplier Award from Bell Helicopter
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As part of the American Society of Materials (ASM), the Materials Education Foundation each year conducts Material Camps as an opportunity for high school aged students to have a hands-on introduction to the field of materials science and engineering. The camp relies on volunteer engineers from industry to help the participants gain a better understanding of what material engineers do.
FTI hosted a group of five students for a day and introduced them to the world of metal fatigue. Len Reid hosted them and they had a chance to "break" two fatigue specimens and do a tensile test on another to help the students solve a problem that they might encounter working as a materials test engineer. They were then given a tour in the FTI Technology Center where they gained firsthand knowledge of FTI products and how they mitigate fatigue problems in real live airplane structures. "It was real cool watching a dog bone coupon break under cyclic fatigue stress" noted one of the students from a local suburban high school.
The ASM Material Camp provides exciting opportunities for young people, encouraging them to pursue careers in materials, science, and engineering. It is also a way for companies like FTI to get involved in community activities. Our VP Technology, Len Reid, is a long standing member of ASM and is the outgoing chairman of the Puget Sound Chapter.
ASM Materials Camp student installs a ForceTec retainer with Len Reid
in the demo room at Fatigue Technology Headquarters (Seattle, WA).
ASM Materials Camp students at Fatigue Technology Headquarters (Seattle, WA)
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· Aviation Expo China | September 21-24, 2011 @ Beijing, China | Booth TBA
· MRO Europe | September 27-29, 2011 @ Madrid, Spain | Booth #267
· AeroTech | October 18-21, 2011 @ Toulouse, France | Booth #310
· ASIP | November 28 - December 2, 2011 @ San Antonio, TX | Booth #4+5
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